The 510/eGo connector on the V5 is identical with V3, so there should be no compatibility issues. One possible exception is the first version V3 that is equipped with brass eGo
threading. If the brass threading has slipped and moved up there could be problems.
sigelei fixed this with one-piece, all stainless steel 510/eGo threading on all V5s and second generation V3s.
There is a huge thread for
sigelei V3 and V5 users here on the Variable Voltage APV Discussion. You will find it on Page 2 right now. It still works.